Ba.sub.3 Al.sub.2 O.sub.6 has long been known to be valuable as an impregnant for porous billets in the manufacture of high current density cathodes. Ba.sub.3 Al.sub.2 O.sub.6 has heretofore been prepared according to the reaction: EQU 3BaCO.sub.3 +Al.sub.2 O.sub.3 .fwdarw.Ba.sub.3 Al.sub.2 O.sub.6 +3CO.sub.2
or to the reaction: EQU 6BaO.sub.2 +2Al.sub.2 O.sub.3 .fwdarw.2Ba.sub.3 Al.sub.2 O.sub.6 +3O.sub.2
The difficulty with the aforesaid reactions is that they involve high temperatures that cause too much impregnant to be used up prior to placing the impregnant in the cathode environment. This cuts down on the life of the cathode. Moreover, the gaseous by-products in the foregoing reactions create vacant holes in the pores of the billet after the impregnant solidifies causing the impregnant to pop out of the cathode environment.